At present, electrically conductive adhesive solid hydrogels and liquid gels are used in the medical device field to provide an electrical interface to the skin of a subject to couple electrical signals into and/or out of the subject (e.g., for diagnostic and/or monitoring uses) and/or to couple electrical stimulus into the subject (e.g., for treatment and/or preventative uses). However, the present hydrogels and liquid gels are inadequate in various aspects.
Prior hydrogels exhibit problems with their adhesive and/or cohesive strength in that they do not sufficiently adhere to the skin, they are insufficiently cohesive to allow for easy removal, and/or they are adherent to themselves such that they must be physically separated, as by a barrier film, to ensure separability (no straight face-to-face, gel-to-gel, configurations). See, e.g., Gilman, et al., U.S. Pat. No. 5,402,884 (a package system providing electrical communication between two hydrogel portions, but still requiring separation of the two hydrogel portions). Additional problems with prior hydrogels concern sufficiently hydrating the skin in contact with the hydrogel and, therefore, problems with sufficiently lowering the skin's electrical resistance thereby frequently resulting in heating to a point of burning the skin upon electrical stimulation. See, e.g., E. McAdams, “Surface Biomedical Electrode Technology,” Int'l Med. Device & Diagnostic Indus. pp. 44 48 (September/October 1990).
Further problems with prior hydrogels include insufficiently wetting around skin hair and resultant problems with insufficiently contacting the skin. This leads to insufficient electrical contact thereby frequently resulting in decreased efficacy of defibrillation and increased incidences of heating to the point of burning the skin upon electrical stimulation and/or problems of requiring preparation of skin surfaces prior to use thereby resulting in slowing the speed of procedures. Further still, electrical pulses transmitted through prior hydrogels to a patient cause hydrolysis of the gel, and this problem is exacerbated with medical stimulation equipment used for defibrillation and/or cardiac pacing because these types of stimulation equipment usually deliver higher voltages and currents to the patient which increases the rate of hydrolysis. For example, defibrillation equipment typically delivers up to 5,000 volts to the patient at a maximum current of 60 amps, and cardiac pacing equipment commonly delivers up to 300 volts to the patient at a maximum current of 0.2 amps.
Yet another problem with prior hydrogels is that the hydrogels often have an unpleasant odor associated with them and are irritating to the skin of a patient. Skin irritation issues and odor often arise where polymerization of the functional monomer and/or other monomeric residues in the hydrogel is not complete. In some cases, other undesired monomeric or other residues are present and, over time after manufacture, may come in direct contact with the patient skin and thus may further cause skin irritation or cause malodor.
Prior hydrogels have attempted to overcome this unpleasant odor and irritation of the skin (caused by lack of polymerization of the monomer) by introducing a solubilizer to enhance the solubility of a polymerization initiator. Although effective, solubilizers are very expensive and are often up to 35 times as expensive as a comparable amount of organic solvent.
Thus, there remains a need to develop a cost effective hydrogel which is not malodorous and does not irritate the skin, while still providing properties of adhesive and cohesive strength and sufficient wetting.